This invention relates to torque shaft assemblies for moving an array of adjustable members to rotate variable stator vanes in a gas turbine engine. More particularly, it relates to damped hollow torque shaft assemblies with damping media inside a hollow interior of the shaft.
Gas turbine engines with variable stator vanes (VSV), for example variable compressor stator vanes, frequently include a torque shaft assembly associated with an actuator. Such an assembly enables and coordinates movement of a plurality of stages of stator vanes responsive to controlled, changing engine conditions by way of crank arms connected to a unison ring for varying the angle of the vanes in each stage. A torque shaft is used to actuate the variable stator vane system of the high-pressure compressors on engines such as the General Electric LM2500+ engine. Generally, a torque shaft actuation system is advantageous in providing flexibility in stage to stage (non-linear) VSV scheduling. Examples of gas turbine engines including axial flow compressors having variable stator mechanisms are disclosed in U.S. Pat. Nos. 2,858,062, 2,933,235, and 5,281,087. An example of a torque shaft assembly is disclosed in U.S. Pat. No. 4,890,977.
Currently used torque shaft assemblies include solid metal shafts upon which are provided features such as recesses, slots, indentations, lugs, etc., on the outer surface of the shaft to receive or provide connections with other assembly parts, for example, tumbuckles. Some torque shafts are crank shafts with cranks or shafts fixedly connected to and disposed between two crank arms and the crank arms are rotatable about an axis of rotation. Because torque shafts can be subject to undesirable flexural (flex) action resulting from engine vibration, some gas turbine engine solid shafts have been provided with a generally central mount, in addition to end mounts, to restrict such undesirable motion of the shaft. Torque shaft assemblies that include crank shafts cannot incorporate such a central mount. Torque shaft assemblies having solid crank shafts with only end mounts have been used. It has been found that premature wear has been observed with such end-mounted torque shafts, sometimes in a very short period of time. For example, such wear has occurred on forward shaft journals and on aft spherical bearings. Means to restrict or change the occurrence of undesirable motion of such end-mounted shafts to avoid premature wear would reduce the need for early repair or replacement of torque shafts.
Wear on one form of solid torque shafts in a gas turbine engine torque shaft assembly for an axial flow compressor variable stator vane assembly has been found to occur due to first flex natural frequencies or forced response (due to engine imbalance) of the solid shaft being driven in an engine operating range at high speed. These modes are driven by the inherent one per revolution (1/rev) balance condition of the compressor rotor. A solid shaft first flex frequency for one type of axial flow gas turbine engine crosses the 1/rev line very close to the operating speed of the engine at maximum power. Since this is the speed where the engine spends nearly all of its operating time, the solid shaft becomes excited and the force of its vibration causes relatively rapid wear of the solid shaft and/or its associated members and support bearings.
One technique to avoid this problem is the use of a hollow shaft in the assembly of the present invention, which would appear to move the 1/rev line crossing well above the maximum engine operating speed for that engine. At the same time, the 2/rev crossing of the first flex frequency modes is maintained at or below engine idle speed. A hollow shaft assembly offers flexibility not available with the solid shaft design to operate at higher natural frequencies and to enable tuning of the shaft frequency within the constraints of the engine mounting points and envelope available.
The axis of rotation of the tube can be moved by adjusting the design of the crank arms so that all attachment points to associated actuator assembly members are outside of the tube and away from the hollow shaft surface. This is done to ensure that the integrity and stiffness of the hollow shaft, in this case a tube, was maintained to maximize frequency. This is in contrast with the assembly using the prior art solid shaft in which at least a part of the attachment points are within the outside diameter of the solid shaft, for example, at indentations or portions machined into the solid shaft. This design substantially reduces the shaft""s frequency.
An exemplary selected outside diameter and different hollow shaft wall thickness should be evaluated to select a desired frequency for the particular engine design. It is desirable to move the 1/rev crossing as high as possible keeping the 2/rev crossing at or below the engine idle speed, generally as discussed above. However, engine tests has shown that a forced response vibratory level of the hollow shaft is still too high if the engine has high core vibration level comparable to the vibratory level of the self-excited solid torque shaft. Analysis supported by component and engine testing has demonstrated that the natural frequency of the torque shaft depends on bearing clearance and forcing amplitude. Additional factors to be considered in designing the torque shaft assembly include the complexity of engine system vibratory mode and the fact that engine vibratory signature may vary slightly between engines. It is highly desirable to eliminate vibratory excitation of the hollow torque shaft and associated assembly at all operating speeds and under all operating conditions.
The present invention, in one form, provides a torque shaft assembly including a tube with a central axis disposed between and fixedly connected to first and second end shafts at first and second distal ends, respectively, of the tube for movement of an array of adjustable members. A hollow interior of tube having a length between the first and second crankshafts is filled with a sufficient quantity of flowable inertia material or damping media to absorb vibratory energy by friction during operation of the engine. A preferred flowable inertia material is round steel shot. The hollow interior is preferably filled with a quantity of damping media to a level in a range of between 85%-98% by volume and a more preferred level of about 98%. Other types of inertia material suitable for use are particulates or pellets such as sand and small plastic spheres, respectively.
A plurality of spaced-apart devises are fixedly attached to a tube wall on an outer surface of the tube wall surrounding the hollow interior. Each clevis includes connection means disposed away from the wall outer surface for connection with an actuator for movement of the array of adjustable members.